The present invention generally relates to apparatus and methods for the selective coating of metal parts using an electrolytic deposition process, as might be advantageous for hardcoat anodizing the heads of aluminum pistons to be used in an internal combustion engine. More specifically, the invention relates to apparatus and methods for continuously processing such metal parts with minimal handling to obtain coatings of superior quality and consistency while simultaneously lessening their relative cost.
Known processes for the selective coating of metal parts, such as the heads of aluminum pistons, generally include a series of timed dips in one or more baths of appropriate cleaning, etching, plating and/or rinsing solutions to generate the desired single or multilayered coating thereon. Complex materials-handling equipment is required to achieve these sequential dips, including equipment for transporting racks containing multiple workpieces between each bath, and for lowering the racks into each bath for a prescribed time period. A large physical plant is thus required to house both the materials-handling equipment and the requisite processing baths, implicating substantial capital investment.
While some processes for producing multilayered coatings may reduce the number of physical "dips" required, as through use of a single bath containing a "mixed" plating solution combined with alternating anodic and cathodic plating steps, as taught in U.S. Pat. No. 3,556,958 to Hutchings et al, there nonetheless remains a substantial need for the handling of individual and/or racked workpieces during such processes. Such handling, in turn, can adversely impact the overall quality and consistency of the resulting coating while further increasing its relative cost, both in terms of increased coating times and equipment/capital requirements.
Indeed, in the event that only a portion of the workpiece is to be coated, as where a coating is sought only on the head of a piston, a further substantial expense is encountered when masking-off those areas which are to remain coating-free, with yet further complications in handling the masked workpieces.
Another problem encountered with known methods for coating metal parts lies in the fact that many of the solutions used therein must be maintained at temperatures other than at ambient temperature. For example, it is well known that the optimal temperature of the sulfuric acid electrolyte used in the hardcoat anodizing of 6000-series aluminum alloys is 32.degree..+-.2.degree. F. (0.degree..+-.1.degree. C.), with the parts perhaps being subsequently dipped into hot water maintained at a temperature of at least 200.degree. F. (95.degree. C.) for perhaps 15 minutes in order to hydrate/seal the resultant aluminum-oxide coating. Similarly, the nominal temperature of a chromic acid electrolyte bath is preferably 100.degree..+-.9.degree. F. (37.degree..+-.5.degree. C.).
Given the open processing tanks typical of such known coating methods, a great deal of energy must be expended in heating or cooling its various solutions to the proper temperature. The latent heat of the sizable racks and other materials-handling equipment used in connection with those methods must also be factored into the energy equation, as must the larger processing tanks and greater quantities of solutions required to accommodate such parts-handling equipment.
Moreover, such known coating methods inherently pose certain safety problems to workers and the environment. In particular, the use by such methods of open processing baths exposes workers to fumes generated either by the solutions themselves or as a byproduct of electrolysis, with the intensity of the fumes almost certainly increased through requisite agitation of each bath. And, since the processing baths must remain open for substantial periods to accommodate the parts-handling equipment, there is little opportunity to recover such fumes and, hence, greater harm to the environment.
Accordingly, what is needed is a semi-automatic method for the selective coating of metal parts, such as pistons, which avoids the aforementioned problems to provide a single or multilayered coating of superior quality and consistency, preferably at a lower cost.